The Microwave Landing System, MLS, is an instrument approach and landing aid system intended to provide an aircraft with its position in spherical coordinates in a reference frame tied to the landing runway, that is to say an angle of azimuth, an angle of elevation and a distance between the landing runway and the aircraft. The distance between the runway and the aircraft is provided by auxiliary equipment for measuring distance known by the acronym DME standing for the expression “Distance Measurement Equipment” and operating on a different frequency or by a global positioning system. The MLS has been developed to alleviate the drawbacks of the Instrument Landing System (ILS) and adopted by the International Civil Aviation Organization (ICAO) to succeed ILS. MLS makes it possible to perform curved and segmented approaches, category I, II and III landings and allows an increase in aircraft landing rates by virtue of a decrease in the spacing between aircraft.
MLS, as standardized by the ICAO, transmits signals for lateral guidance, that is to say an angle of azimuth, and for vertical guidance, that is to say an angle of elevation, by using a time-referenced scanning beam technique and a time division multiplexed signal. The use of a time division multiplexed signal allows the transmission of the lateral and vertical guidance signals on the same radiofrequency channel without creating interference between the lateral guidance signals and the vertical guidance signals. The guidance signals are emitted on a frequency of around 5 GigaHertz (GHz) by an azimuth station and an elevation station. The azimuth station is placed at the end of the runway while the elevation station is situated on the side of the runway, about 300 meters (m) from the start-of-runway threshold. Each station transmits a narrow scanning beam sweeping the space of coverage to and from in outward and return fashion at constant angular speed following the relevant angular coordinate. An antenna and a receiver on board the aircraft receive the scanning beam a first time during the outward sweep and a second time during the return sweep. It is thus possible to determine the angle of azimuth and the angle of elevation through the following linear relation:
                    θ        =                              (                          T              -                              T                0                                      )                    ·                      v            2                                              (        1        )            where θ is the angle of azimuth or the angle of elevation,
T a time interval between the reception of the outward and return passes of the scanning beam,
T0 the value of the time interval T for a zero angle θ and
v the angular sweep rate.
T0 and v are constants defined by the international standards on MLS.
The microwave landing system with computed axial approach, called MLS-cc, the acronym standing for the expression “Microwave Landing System—Computed Centerline”, is an MLS in which the azimuth station is not placed at the runway extremity but is offset to one side of the landing runway. The displacement of the azimuth station can notably be used in two typical cases. In the first case, the azimuth station is situated in proximity to the elevation station for the sake of simplicity of deployment of the MLS. This configuration is encountered mainly in the case of tactical equipment deployed on makeshift and unprepared strips. In the second case, the MLS-cc is used for the approach on a secondary runway not equipped with an MLS but situated in the zone of coverage of the runway equipped with the MLS. In both cases, on account of the offset of the azimuth station, the aircraft's receiver measures an angle, called the real azimuth angle, which does not correspond to the angle of azimuth in the conventional sense of the term, called the virtual azimuth angle. It is therefore necessary to compute the virtual azimuth angle so as to be able to provide the pilot with an item of information which is recentered with respect to the runway axis. To evaluate it, it is necessary to compute the position of the aircraft in a Cartesian reference frame centered on a ground reference point, called the ground point. This ground point is for example one of the two stations or the point of intersection between the runway axis and a straight line perpendicular to the runway axis and passing through the elevation station. This intersection point is called the MLS datum point.
The computation of the position of the aircraft is carried out through a system of 3 equations with 3 unknowns, parametrized by the real azimuth angle, the angle of elevation and a distance between the aircraft and the ground point. These equations being non-linear, iterative algorithms are used to solve the system. Conventionally, the iterative algorithms are of the Gauss-Seidel or Newton-Raphson type. By using a satellite positioning system, it is possible to use the MLS datum point as ground point for the determination of the distance to the aircraft. However, there are risks related to the behaviors of the iterative algorithms in this situation, as set out in the standard DO-226.
With the aim of optimizing the speed of convergence to the position of the aircraft and the precision of this position, two iterative algorithms can be used in succession, the first to converge quickly around the position of the aircraft, the second to obtain better precision of this position. However, such a combination of algorithms presents the drawback of complicating the determination of the position of the aircraft, making it difficult to set up the iterative algorithms and to validate them. Moreover, these algorithms are slow to execute, expensive in terms of computational load and behaviorally risky, in particular on account of the risks of multiple solutions, divergence and stationarity. Finally, these algorithms degrade the computational precision through their iterative nature, in particular the propagation of errors.
The iterative algorithms are executed by the aircraft's receiver, for example a multimode receiver (MMR). The MMR comprises a radiofrequency chain, a digital signal processor (DSP), a global positioning system (GPS) receiver, and a microprocessor. The radiofrequency chain receives signals originating from various systems, in particular the guidance signals originating from the azimuth station and from the elevation station. The processor processes the signals originating from the radiofrequency chain so as to extract the angles of azimuth and of elevation but also auxiliary words contained in the guidance signals. The GPS receiver can be internal or external. It transmits the airplane's positions. The microprocessor fulfills several functions, including computation of the distance between the MLS datum point and the aircraft, computation of the position of the aircraft on the basis of the iterative algorithms, computation of deviations between the position of the airplane and an optimal descent axis, management of the equipment and communication interfacing with a link of an airplane bus, for example an ARINC bus, the abbreviation for the “Aeronautical Radio Incorporated” series of standards. The microprocessor comprises a device for managing the algorithms making it possible to detect and process the divergence, non-convergence or false convergence of an algorithm, to manage the initialization, sequencing and combining of the algorithms.